Field of the Invention
The present invention concerns the generation of x-rays, and particularly time-resolved x-rays having picosecond and shorter duration.
The present invention relates to the production of x-ray radiation, particularly time-resolved pulses of x-ray radiation.
The earliest attempts to produce time-resolved x-rays employed mechanical shutters that moved in front of x-ray sources. For example, transmission of x-rays through x-ray transparent apertures within a rotating apertured disk that was otherwise opaque to x-rays permitted the generation of millisecond x-ray pulses. These millisecond x-ray pulses were too slow to permit the study by x-ray diffraction of any type of molecular phenomena such as reaction, melding, dissociation, or vibration. Millisecond x-ray pulses were, however, sometimes sufficient to permit observation of certain biological phenomena, although not normally at the biomolecular level.
Davanloo et al., Rev. Sci. Instrum. 58:2103-2109 (1987) reported constructing an x-ray source capable of producing x-ray pulses of nanosecond (ns) duration. That x-ray source utilized (i) a low impedance x-ray tube, (ii) a Blumlein power source, and (iii) a commutation system for periodically applying power from the Blumlein power source to the x-ray tube. The system yielded 140-mW average power in 15 ns pulses of radiation near 1 .ANG.. That device, and others based on Blumlein-generators, suffers from (i) low repetition rates in the range of 100 hertz, (ii) prospective inability to produce pulses shorter than about 15 nsec, and (iii) low energy efficiency on the order of 25%. The durability in operational use of Blumlein-based sources of x-ray flashes is also uncertain.
More recently, Science News, Vol. 134, No. 2: pp. 20 (1989) reported that scientists at Cornell University and the Argonne National Laboratory have developed a device, called an undulator, capable of producing x-ray pulses one-tenth of a billionth of a second (100 picoseconds) in duration. The undulator utilized synchrotron radiation from fast-moving charged particles in an electron storage ring. Because electron storage rings are typically large and expensive, the ring used at Cornell being one half-mile in diameter, the production of bright x-ray flashes by such means is distinctly not adaptable to the scale and budget of a typical materials or biological laboratory.
X-rays have been produced using plasma sources that are energized by lasers. In laser plasma x-ray sources, either a pulsed-infrared (IR) laser or a ultraviolet (UV) excimer laser is used with pulse widths varying from 50 picoseconds to 10 nanoseconds. The laser beam is focused on a target where it creates a plasma having a sufficiently high temperature to produce continuous and characteristic x-ray radiation. Major disadvantages of laser plasma x-ray sources include (i) a diffuse, non-point, area of x-ray emission and (ii) low efficiency.
By way of background to the present invention, Lee, et al., in Rev. Sci. Instrum., 56:560-562 (1985) described a laser-activated photoemissive source of electrons. In the laser-activated photoemissive electron source a photocathode is illuminated with high intensity laser light as a means of generating numerous electrons by the photoelectric effect. The electrons emitted from the photocathode are focused in an electrical field, typically produced by electrodes in an electron-gun configuration, in order to produce a high intensity electron beam.
By way of further background, the rectification of ultrashort optical pulses in order to generate electrical pulses having durations and amplitudes that are unobtainable by conventional electronic techniques is described by Auston, et al. in the Annl. Phys. Lett., 20:398-399 (1972). Electrical pulses on the order of 4 amperes in 10 picoseconds are generated by rectification of 1.06 micrometer optical pulses in a LiTaO.sub.3 crystal doped with approximately 2.24% Cu (LiTaO.sub.3 :Cu.sup.++).
A doped transmission line, having an absorption coefficient of 60 cm.sup.-1 and a thickness of 0.2 mm, is bonded with a thin epoxy layer to an undoped crystal in the form of a TEM electro-optic transmission line of 0.5.times.0.5-mm cross-sectional area. Current pulses are generated by absorption in this transducer of single 1.06 micrometer mode-locked Nd: glass laser pulses, typically of duration 3-15 psec and with an energy of approximately 1 mJ.
The electro-optic transmission line, or switch, operates to conduct current during the presence of laser excitation by action of the macroscopic polarization resulting from the difference in dipole moment between the ground and excited states of absorbing Cu.sup.++ impurities. Effectively, the electric-optic transmission line, or switch, has a very great number of charge carriers, and is a very good conductor, during the presence of laser excitation. During other times it is a semiconductor and does not conduct appreciable current. The excited-state dipole effect of the transmission line, or switch, is exceptionally fast, on the order of 1 or 2 psec or less.